This invention relates to a combined oven which operates both on microwave heating and electric resistance heating and which further has a ventilating system as well as a device for the pyrolytic cleaning of the cooking chamber. The bottom of the cooking chamber is provided with a microwave window which is tightly covered by a glass-ceramic plate and below which there are arranged a microwave connecting housing and devices for generating and guiding the microwave energy.
An oven of the above-outlined type is conventional and is disclosed, for example, in German Offenlegungsschrift (Laid-Open Application) No. 2,715,655. It is well known that during baking or broiling by means of electric resistance elements, a significant amount of vapor mist is generated which penetrates through the smallest cracks and then, by precipitation, soils the oven components. Particularly in case of sensitive electric and electronic components such occurrences may adversely affect the operational reliability of the oven to a significant degree. In order to avoid such condensation deposits in the extremely sensitive devices that generate and guide the microwave energy, the glass-ceramic plate which covers the window provided in the bottom of the cooking chamber is secured to the bottom with the intermediary of a circumferential, high-temperature resistant seal. It has been found, however, that during normal operation of the cooking chamber such a seal absorbs vapor mist and gases and first stores the precipitated grease particles and the like. If then the cooking chamber of the oven is periodically submitted to a pyrolytic heat cleaning cycle during which, as known, temperatures in excess of 500.degree. C. may be generated, such high temperatures cause the grease and similar material absorbed in the seal of the glass-ceramic plate to vaporize and thus escape from the seal. The grease vapor or other impure gases emanating from the seal escape in all directions and while doing so, also reach the microwave generating and guiding system underneath the glass-ceramic plate. There the grease vapor condensates and accumulates on sensitive components, such as the magnetron and the antenna of the microwave energy generating device. Over a period of time the precipitated particles lead to a soiling of the components to such an extent that a reliable operation is endangered and a premature breakdown of the apparatus can be expected.